Amine salts having anions derived from volatile acids are produced as by-products from a number of industrially important reactions. For example, N-aryl-N',N'-dialkylureas can be cracked in the presence of hydrochloric acid (U.S. Pat. No. 3,898,259), organic sulfonic acids (U.S. Pat. No. 4,883,908), tertiary amine hydrohalide salts (U.S. Pat. Nos. 4,871,871 and 4,873,364), and other promoters to give aryl isocyanates. Dialkylammonium salts are typically the by-products from such reactions. In these urea cracking processes, and in other industrial processes that generate amine salts, it is desirable to recover both the amine and the volatile acid from the ammonium salt for reuse in the process.
Unfortunately, few economical and satisfactory methods for recovering amines and acids from amine salts are known. Most of the known methods focus on recovery of a valuable amine from an amine salt by reacting the amine salt with a less valuable amine or an alkali metal or alkaline earth metal hydroxide or carbonate in solution. For example, U.S. Pat. No. 3,471,562 teaches a process for making aliphatic tertiary amines by reacting a tertiary amine hydrohalide salt with an aqueous metal hydroxide to liberate the tertiary amine. Unfortunately, such processes require costly disposal of inorganic by-product wastes. U.S. Pat. No. 3,849,496 teaches a method for preparing amines by heating an amine hydrohalide with ammonia in an organic solvent. In this process, a valuable amine is recovered, but another amine salt is generated.
A conceptually desirable way to recover both acid and amine from amine salts is to thermally decompose the salt. Unfortunately thermal decomposition is not practical if a volatile acid is generated because the acid and amine rapidly recombine in the vapor phase to regenerate the salt.